Home > Storage > PowerScale (Isilon) > Product Documentation > Data Protection > Data Protection with Dell PowerScale SnapshotIQ > Reading from a snapshot
The following figure shows three snapshots of a given file with logical inode (LIN) value “1:abcd:1234.” In this case, blocks 3 and 4 were changed after the first snapshot (Snap_ID 98) was taken and before the second (Snap_ID 100), and blocks 0 and 4 were changed after the second snapshot was taken.
When the data is not in the snapshot, the block tree of the inode on the snapshot does not point to a real data block. Instead it has a flag marking it as a “Ditto Block.” A Ditto-block means that the data is the same as the next newer version of the file, so OneFS will automatically look ahead to find the newer version of the block.
The arrow represents the logic for reading block 2 from snapshot 98. Since it was not changed in snapshot 98, the read has to fall forward to snapshot 100. It was not changed there either, so the write falls forward to the head version of the file.
The performance implications here should be clear: If you have thousands of snapshots of the same unchanged file, reading from the oldest snapshot can potentially be somewhat slow.